It has been progressing to develop a system that detects an obstacle around a vehicle by utilizing an ultrasonic wave or the like. For example, this type of system includes an ultrasonic sensor having a generation (output) function for generating an ultrasonic wave, and a reception (detection) function for receiving an ultrasonic wave, and disposed at multiple positions on a bumper of a vehicle. When the sensor receives an ultrasonic wave generated from the sensor, the sensor recognizes the received ultrasonic wave as a wave reflected on an object around the vehicle. In this case, the sensor calculates a distance to the object based on a time from generation to reception, and notifies a driver about the presence of the object when the calculated distance is a predetermined distance or shorter, for example.
A technology capable of reducing misdetection by an ultrasonic sensor may be important. For example, Patent Literature 1 identified below discloses an ultrasonic sensor that outputs an ultrasonic wave by oscillating an oscillator of the ultrasonic sensor at a frequency different from a resonant frequency of the oscillator, and determines that the ultrasonic sensor is in an abnormal condition when a frequency of a wave reception signal corresponds to the resonant frequency.
The inventors of the present invention have found following current circumstances. It has been pointed out that the foregoing system for detecting an obstacle around a vehicle causes misdetection by regarding a raindrop or the like falling on and adhering to the sensor as reception of a reflected wave in some cases. Raining or like events are possible and frequent phenomena, and therefore this type of misdetection considerably lowers reliability of the obstacle detection system. Accordingly, there has been a strong demand for a system capable of reducing this type of misdetection.